/Engine/Source/Runtime/SlateCore/Public/Fonts/FontCache.h
The indices used here are relative to the start of the text we were shaped from, even if we were only shaped from a sub-section of that text
The sub-sequence, or an null if the sub-sequence couldn't be created (eg, because you started or ended on a merged ligature, or because the range is out-of-bounds)
FShapedGlyphSequencePtr GetSubSequence ( const int32 InStartIndex, const int32 InEndIndex ) const